Elucidating the etiology of onion bacterial scale rot in the semi-arid region of Northeastern Brazil

The etiology of onion scale rot caused by bacteria in the semi-arid region of northeastern Brazil is unclear, which complicates disease control. Forty-five bacterial strains collected from the main onion producing regions in the states of Pernambuco and Bahia were identified by sequencing and phylogenetic analysis of the 16S rRNA gene and were characterized by rep-PCR and pathological behaviour on onion. The strains were grouped into three clades: 29 strains in clade I formed by the Burkholderia cepacia complex, 10 strains in clade II formed by Burkholderia gladioli, and six strains in clade III formed by Pseudomonas aeruginosa. Rep-PCR analysis grouped the strains in 31 clusters at 70% similarity. However, it was not possible to identify the three clades by rep-PCR analysis. Inoculation of wounded onion scales showed that the strains from the B. cepacia complex and B. gladioli were more aggressive on onion scales than those from P. aeruginosa. Based on the phylogenetic identification performed in this study, we conclude that scale rot of onion bulb in the semi-arid region of northeastern Brazil is caused by bacteria from the B. cepacia complex, B. gladioli, and P. aeruginosa. In addition, more than one species of B. cepacia complex may be associated with the disease in this region.